B3. Climate-vegetation-soil Interactions and Long-term Hydrologic partitioning: Signatures of Catchment Co-evolution?

Abstract

Catchment hydrologic partitioning, regional vegetation composition and soil properties are strongly affected by climate (Budyko, 1974; Whittaker, 1962; Jenny, 1941), but the effects of climate-vegetation-soil interactions on river basin water balance is still poorly understood. Here we use a physically-based hydrologic model applied to 12 US catchments across a climate gradient to decouple climate and landscape properties in an attempt to gain insight into the role of climate-vegetation-soil interactions in long-term hydrologic partitioning. The 12 behavioral catchment models are subjected to the 12 different climate forcings, resulting in 144 10-year model simulations. The results are analyzed per catchment (one catchment model subjected to 12 climates) and per climate (one climate filtered by 12 different models), and compared to water balance predictions based on Budyko’s hypothesis (E/P=(EP/P); E: evaporation, P: precipitation, EP: potential evaporation). We find significant anti-correlation between average deviations from predicted evaporation index (E/P) computed per catchment vs. per climate. Catchments that on average produce more E/P have developed in climates that on average produce less E/P, when compared to Budyko’s prediction. Water and energy seasonality could not explain these observations, confirming previous results reported by Potter et al. (2005). Next, we analyze which model (filter) characteristics explain the catchment’s tendency to produce more or less E/P. We find that the time scale that controls perched aquifer storage release explains the observed trend. This time scale combines several geomorphologic and hydraulic soil properties. Catchments with relatively longer aquifer storage release time scales produce significantly more E/P. Vegetation in these catchments have longer access to this additional groundwater source and thus will be less prone to water stress. Further analysis reveals that climates that give rise to more (less) E/P are associated with catchments that have vegetation with less (more) efficient water use parameters. In particular, the climates with tendency to produce more E/P have catchments that have lower % root fraction and less light use efficiency. Our results suggest that their exists strong interactions between climate, vegetation and soil properties that lead to specific hydrologic partitioning at the catchment scale. This co-evolution of catchment vegetation and soils with climate needs to be further explored to improve our capabilities to predict hydrologic partitioning in ungaged basins.